A. Field of the Invention
This invention relates generally to the field of aerial reconnaissance photography and camera systems used for such photography. More particularly, the invention relates to an optical system forming an objective lens subassembly for an aerial reconnaissance camera. The optical system is particularly suitable for use in a dual band framing reconnaissance camera, in which the optical system directs radiation in different bands of the electromagnetic spectrum simultaneously onto two separate two-dimensional image recording media which generate frames of imagery of the scene simultaneously.
B. Description of Related Art
Long Range Oblique Photography (LOROP) cameras have been developed as a result of the need to obtain clear, high resolution pictures from longer ranges, typically from 10 to 50 nautical miles from the terrain of interest. The advent of LOROP cameras was an outgrowth of development of weapons technology, which could engage reconnaissance aircraft at ever-increasing distances, and geopolitical boundaries that became more and more difficult to encroach upon.
With the advent of LOROP cameras came the operational intricacies of using very sensitive and high performance instruments in a fashion that yielded the intelligence, i.e., image resolution, required of them. These operational issues were hostage to the technological limitations of the day. Initially, all cameras were film. Film LOROP cameras have been operated both as panoramic scanning (line scan) and framing cameras. Panoramic scan cameras collect an image with a smooth rolling motion of the camera while exposing film by pulling it passed a slit. The advantage of this approach was ease of implementation of the scanning mechanism. The disadvantage is that each line of exposed imagery was taken from a different perspective, hence the scanning system inherently was prone to creating geometrically and geospatially distorted images.
Subsequently, LOROP film framing cameras were employed. These cameras captured a frame of imagery by rapidly moving a slit across the film for exposure. The cameras utilized a scan head mirror assembly that could be moved in order to take successive frames of imagery at a selected depression angle relative to the horizon, depending on the target location.
Later, electro-optical line scan cameras entered the market as a filmless solution. Instead of film, the cameras used a solid state linear line scan charge coupled device (CCD) as a detector. These cameras used a scan mirror or the motion of the ground below the aircraft to scan the image across the line of photosensitive detectors that made up the CCD to form a frame, line by line. Again, the disadvantage of this method was that imagery was obtained from a different perspective as the aircraft moved, resulting in geometrically and geospatially distorted images.
Step framing cameras were developed which take a full frame of imagery at one time, then step the camera to a new angular position, take the next frame of imagery (with some overlap between the images to insure 100% coverage), step and generate a new frame of imagery, and so on until the desired scene is covered. The disadvantage of step framing cameras was that the stepping action was very difficult to accomplish with the whole camera, therefore it had to be broken into a scan head that performed the stepping and an image de-rotation mechanism, both of which were tied together by a synchronized drive system. The advantages of step frame cameras as compared to line scanning cameras are higher geometric fidelity and geospatial accuracy. Originally, full framing cameras were all film.
The next revolutionary step in the art of LOROP and tactical aerial reconnaissance cameras was the development of two-dimensional area array electro-optical (E-O) detectors. This occurred several years after the electro-optical linear arrays were first developed, and required semiconductor processing technology to mature many more years before such arrays were practical for reconnaissance use. Recon/Optical, Inc., the assignee of the present invention, in the early 1990""s, introduced large area focal plane arrays to the reconnaissance industry. One such array is described in U.S. Pat. No. 5,155,597 to Andre G. Lareau et al., the contents of which are incorporated by reference herein. Such cameras were the first large area arrays to be used in tactical aircraft, as well as strategic reconnaissance aircraft such as the high altitude SR-71 aircraft. These large area arrays had the advantage of providing an image from a single point in space giving excellent geometric fidelity. Moreover, the high pixel count, and optimal pixel size, allowed such cameras to produce imagery having outstanding image resolution.
Furthermore, as described in the ""597 Lareau et al. patent, it was possible to perform forward motion compensation in side oblique, forward oblique and nadir camera orientations electronically. U.S. Pat. No. 5,668,593, also to Lareau et al., describes a step-frame electro-optic camera system with electronic forward motion compensation. U.S. Pat. No. 5,798,786, also to Lareau et al., describes a method for compensation for roll, pitch or yaw motions of an aerial reconnaissance vehicle, in addition to forward motion compensation, electronically in the focal plane of an E-O detector. The ""593 and ""786 Lareau et al. patents are incorporated by reference herein.
Framing E-O LOROP camera systems were a logical platform to host the advanced detectors such as described in the Lareau et al. ""597 patent. Electro-optical detectors, such as described in the Lareau et al. ""597 patent, are capable of being fabricated from selected materials that can detect incident radiation in a variety of portions of the electromagnetic spectrum, and not just the visible spectrum. In particular, the advantages of large area framing can be enhanced by providing imaging capability in the infrared (IR) portion of the spectrum. A camera that generates frames of imagery in two distinct portions of the electromagnetic spectrum simultaneously is referred to herein as a xe2x80x9cdual band framing camera.xe2x80x9d The patent to Gilbert W. Willey, U.S. Pat. No. 5,841,574, also assigned to Recon/Optical, Inc., describes a multi-spectral, decentered aperture, catadioptric optical system particularly suitable for a dual band line scanning camera system having two linear electro-optical detectors, one for the visible or near IR (xcex=0.5 to about 1.0 microns), and one for either the mid-wavelength IR (xcex=about 3.0 to about 5.0 microns) or the long-wavelength IR (xcex=about 8.0 to about 14.0 microns).
The technological capability of dual band framing LOROP cameras promises performance heretofore unavailable anywhere. However, the implementation of such a camera presents a number of difficulties and technical challenges beyond those posed for prior art systems. These challenges are optical, servo-mechanical and operational, and are discussed in further detail below. The present invention provides a dual band framing aerial reconnaissance camera system that overcomes these challenges and difficulties to provide an advanced, high resolution framing camera system that generates imagery of a scene of interest at two different bands of the electromagnetic spectrum.
A dual band optical system for a framing aerial reconnaissance camera is described, wherein the camera includes at least two two-dimensional image recording media for generating frames of imagery of a scene external of an aerial reconnaissance vehicle carrying the camera. The optical system comprises a Cassegrain objective optical subsystem forming an objective lens for the optical system. The Cassegrain optical system receives incident radiation from the scene and comprises a primary mirror having a central aperture and a secondary mirror, the primary and secondary mirrors aligned along an objective optical axis.
An azimuth mirror is provided which receives radiation from the secondary mirror. The azimuth mirror is placed in front of the central aperture of the primary mirror and directs radiation in a direction away from the objective optical axis. A spectrum dividing element receives radiation from the azimuth mirror. This element directs radiation in a first band of the electromagnetic spectrum into a first optical path and directs radiation in a second band of the electromagnetic spectrum into a second optical path different from the first optical path.
The first two-dimensional image recording medium is placed in the first optical path and a second two-dimensional image recording medium is placed in the second optical path. The first and second image recording media generate first and second frames of imagery in two different portions of the electromagnetic spectrum simultaneously.
The aerial reconnaissance vehicle defines a roll axis. In the illustrated embodiment the first and second paths are oriented and extend for a spatial extent generally in the direction of the roll axis, and the objective optical axis is substantially orthogonal to the roll axis, whereby the dual band optical system provides a compact arrangement the camera.
The camera further includes a servo-mechanical subsystem. This subsystem includes a first motor system coupled to the camera housing that rotates the entire camera housing (including the optical system as recited above) about a first axis. The camera housing is installed in the aerial reconnaissance vehicle such that this first axis of rotation is parallel to the roll axis of the aerial reconnaissance vehicle (referred to herein for simplicity as xe2x80x9cthe roll axisxe2x80x9d). The image recording media are exposed to the scene to generate frames of imagery as the first motor system rotates the camera housing in a continuous fashion about the roll axis. The first and second image recording media have a means for compensating for image motion due to the rotation of the camera housing. In an electro-optical embodiment of the image recording media, the roll motion compensation means is preferably comprised of electronic circuitry for clocking or transferring pixel information through the electro-optical detectors at a uniform rate substantially equal to the rate of image motion due to camera rotation. A method of calculating the image motion rate, and thus pixel information transfer rate, due to roll of the camera housing is disclosed herein. If a film camera is used for the image recording media, a mechanical system is used to move the film at a rate substantially equal to the image motion rate.
The servo-mechanical subsystem also includes a second motor system coupled to the objective optical subassembly. In the illustrated embodiment, the objective optical subassembly comprises a catoptric Cassegrain optical system. The second motor system rotates the Cassegrain optical system about a second axis in the direction of forward motion of the reconnaissance vehicle to compensate for the forward motion of the aerial reconnaissance vehicle. The action of the first motor assembly to rotate the entire camera housing about the roll axis occurs at the same time (i.e., simultaneously with) the action of the second motor system to rotate the Cassegrain optical system in the line of flight to accomplish forward motion compensation. The net effect of the action of the Cassegrain motor system and the roll motion compensation system is that the image of the scene of interest is essentially frozen relative to the focal plane of the image recording media while the media obtain the frames of imagery, allowing high resolution images of the scene in two different bands of the spectrum to be obtained simultaneously. Furthermore, the rotation of the image scene caused by the roll motion of the objective subassembly is simultaneously detrotated by the roll motion of the rest of the camera, in view of the fact that the entire camera assembly is rolled as a unit, thereby eliminating the need for a separate derotation mechanism such as a pechan prism. Other types of optical arrangements for the objective optical subassembly are possible, but are less preferred. The operation of the camera with the different type of objective subassembly is the same.
In a preferred embodiment, the first and second image recording media comprise two dimensional area array electro-optical detectors. One may be manufactured from materials sensitive to radiation in the visible and near-IR portion of the electromagnetic spectrum, and in a preferred embodiment is a charge-coupled device (CCD) detector of say 5,000xc3x975,000 pixels. The other of the electro-optical detectors is made from a material sensitive to radiation in the infrared portion of the electromagnetic spectrum, and may be a platinum silicide array of photo diode detectors or other suitable type of electro-optical detector suitable for IR detection. The reader is directed to U.S. Pat. No. 5,925,883 to Woolaway, III, the contents of which are incorporated by reference herein, for a description of an IR detector. The detector sensitive to radiation in the infrared portion of the electromagnetic spectrum is preferably sensitive to radiation having a wavelength of between 1.0 and 2.0 microns (SWIR), 3.0 and 5.0 microns (MWIR), or from about 8.0 to about 14.0 microns (LWIR). In either of the embodiment of electro-optical detectors, they will typically comprise an array of pixel elements arranged in a plurality of rows and columns. The means for compensation for roll motion of the camera housing comprises electronic circuitry for transferring pixel information in the electro-optical detectors from row to adjacent row at a pixel information transfer rate (uniform across the array) substantially equal to the rate of image motion in the plane of the electro-optical detectors due to roll of the camera housing. The transfer of pixel information occurs while the pixel elements are integrating charge representing scene information. Thus, the roll motion compensation can be performed electronically on-chip.
As a further possible embodiment, electro-optical detectors with the capability for transferring pixel information in both row and column directions independently, such as described in Lareau et al., U.S. Pat. No. 5,798,786, could be used for the image recording media. Forward motion compensation and roll motion compensation could be performed on-chip in the detectors.
As noted above, the present invention required the solution to several difficult technical challenges, including optical, servo-mechanical and operational difficulties. For an electro-optical framing LOROP camera to operate in at least two discrete bands of the electromagnetic spectrum at the same time, the optical challenge is to focus panchromatic energy (e.g. visible through IR) on a focal plane detector with (1) good image quality and satisfactory modulation transfer function, (2) while baffling stray energy, (3) meeting space constraints, and (4) enabling the use of a relatively large two-dimensional area array as a focal plane detector to get an adequate field of view and resolution. In accordance with one aspect of the invention, these optical challenges were solved by a unique catoptric Cassegrain objective optical subassembly incorporating an azimuth mirror and utilizing separate field optics for each band of the spectrum, described in more detail herein.
The catoptric Cassegrain type of objective optical subassembly does not lend itself to the use of servo-mechanical systems developed for prior art LOROP systems, particularly those used in prior art step frame cameras (such as described in the Lareau et al. ""593 patent). The prior art step frame cameras use a stepping mirror to step across the line of flight and direct radiation onto the array, and require a de-rotation mechanism, such as a Pechan prism, to de-rotate the images. The standard solution of stepping the entire LOROP camera system or even a large scan mirror assembly at the operational frame rate are not acceptable alternatives for large LOROP cameras, an in particular large dual band systems. In particular, the applications of the present invention are flexible enough to include both strategic and tactical aircraft, as well as the new breed of aircraft being used by the military for reconnaissance known as unmanned aerial vehicles (including low observables). The diversity of these applications posed a power and stability problem that prevents application of prior art solutions. The task of stepping a 400 lb. camera mass two to four times a second creates tremendous inertial loads as well as power spikes that would be unacceptable. Even the inertia and associated settling times of a stepped scan head assembly pose problems in some applications.
This servo-mechanical situation required a unique inventive solution, described in detail herein. The solution, as provided in one aspect of the present invention, was to (1) rotate the entire camera (including the entire optical system and the image recording media) smoothly in a continuous fashion about an axis parallel to the aircraft roll axis, similar to the pan-type movement, but without the starts and stops used in a traditional step-frame camera system, and (2) operating the camera as a framing camera while the camera undergoes the smooth rotation. Frames of imagery are thus taken while the camera smoothly rotates about the roll axis at a constant angular velocity. In addition to this novel xe2x80x9croll-framingxe2x80x9d technique, the illustrated embodiment also electronically compensates for, i.e., stops, the image motion due to roll while the camera is scanning in a smooth motion. Meanwhile, a novel forward motion compensation technique is performed by the Cassegrain optical subassembly to cancel out image motion effects due to the forward motion of the aircraft. The result enables exposures of the image recording media to the scene while compensating for roll and forward motion, enabling high-resolution images to be obtained.
The present invention thus solves the difficult optical, servo-mechanical and operational problems and provides a dual band framing electro-optical LOROP camera that delivers a performance and technical capability that has never before been achieved. In particular, it provides a system by which high-resolution frames of imagery in two different portions of the electromagnetic spectrum can be generated simultaneously. The inventive camera can be used in a quasi-stepping mode, in which overlapping frames of imagery are obtained across the line of flight. It can also be used in a spot mode, in which the camera is oriented in a particular direction to take an image of a target expected to be in the field of view.
Many of the teachings of the present invention are particularly applicable to a dual band electro-optical framing reconnaissance camera, and such a camera is the preferred embodiment. However, as explained below, some of the techniques and methods of the subject camera system, such as the roll-framing operation and unique roll and forward motion compensation techniques, are applicable to a camera system that images terrain in only one portion of the electromagnetic spectrum. Thus, in an alternative embodiment the camera is basically as set forth as described above, except that only a single detector is used and the spectrum-dividing prism and second optical path are not needed. Furthermore, while a preferred embodiment uses a two-dimensional electro-optical imaging array for the detector in each of the bands of the electromagnetic spectrum, the inventive camera system can be adapted to use film or other types of detectors for the photosensitive recording medium. In the film camera embodiment, roll motion compensation could be performed by moving the film in a manner such that the film velocity substantially matches the image velocity due to roll.
While the foregoing summary has described some of the highlights of the inventive camera system, further details on these and other features will be described in the following detailed description of a presently preferred embodiment of the invention.